Pop-off Tester Questions

[Frank_Hassa]

I've read several threads about pop-off testers on this forum and another one that supposedly gives directions on how to build one, but none of it is gelling in my head and I have more questions now after I've done all this reading and youtube watching than I had when I started.

First, can someone please explain both the pop-off testing procedure and also what function it's supposed to test. From what I've read, it has something to do with the pressure at which some part of the carburetor either admits or restricts fuel into the engine and I don't know which and I don't know why, and I don't know why this "pressurizing" "on/off" is function is necessary because to my knowledge lawn mower carbs and automotive/truck carbs do not have this function, and I wonder if there's something unique about boats that requires it? And also if this pressure dynamic might have something to do with a "fuel return line" which also seems odd and unfamiliar in terms of "method" of fuel delivery. So if someone could give a "big picture" description of how the whole thing fits together and why, I'd appreciate it.

Also in terms of the test procedure itself, I'm confused, mostly I think because I don't understand the "big picture" mentioned above, and so maybe if I understood that, the references I've seen to the test procedure/method might make more sense.

Finally the home made test kit. I read a "how to" thread on another forum but the writing clarity was so poor I didn't understand a thing about how to build one. And again, maybe if I understood the whole thing better, building one might seem more self-explanatory. I understand that a gauge is needed, so I wonder if the gauge I plan on purchasing for the compression testing can also be used for the home-built pop-off tester.

Here's the gauge I'm going to buy:
https://www.walmart.com/ip/Equus-3612-Innova-Compression-Tester/14644667

Will this also work for the pop-off tester, and if so, what other pieces and parts do I need?

 

[soccerdad]

I think I pretty much explained the fuel return loop purpose in another of your posts. The pump always pumps more than you need so it has to go somewhere... The popoff test the pressure that it takes to open the needle from the seat. It has to do with the low speed fuel circuit transition. You need to forget almost all you know about motorcycle and lawnmower carbs. These are performance pumper carbs and they are different. Please read http://www.mikuni.com/pdf/sbn_manual.pdf It will tell you all about pop-off. I also had posted this for you on your carb kit post... Not trying to be a d!ck, but you need to read the stuff we post as we are trying to help. Good luck.

 

[soccerdad]

FWIW, I think that compression gauge will not be accurate enough. You need something closer to the 0-50 PSI range. You just need a source of compressed air, or a pump that can slowly be increased a couple PSI at a time. Then you just need a gauge in the line between the air souce and the carb. So you can use whatever fitting you need to make that happen. Dozens of ways to do it. I have an air compressor so I just put a regulator on the hose from the compressor. I have a hose from the regulator connected to the carb using 1/4" fuel line. I have a T in the line for my gauge. (I use an electronic gauge, but a regular cheap gauge will work). So set up the carb with the tester, and the other port blocked off (per the test directions) and I then just slowely increase the pressure until I get it to pop. Do it several times and keep the needle wet with something like WD40. Adjust the spring arm until you get what you want, and that is it. My popoff tester is shown in the Shamu thread.

 

[Sportster-2001-951C-Stock]

to my knowledge lawn mower carbs and automotive/truck carbs do not have this function, and I wonder if there's something unique about boats that requires it?

Ah, you've stumbled onto the diaphragm carburetor but I expect it's likely you've actually experienced this design before, you just don't recall.

Automotive and lawnmower carburetors aren't designed to operate in any orientation as you probably realize, the diaphragm carburetor is capable of operating in any orientation including upside down.. Consider what happens if you flip an automotive carburetor upside down during operation, all the fuel drains out of the bowl and spills onto the ground. Not a good thing to happen in the bilge of a jetski, that would be a nice way to get a fire going and roast your buns.

A few examples of where the common float bowl carburetor won't work well are jet skis (frequently operate upside down), gasoline weed eaters and chainsaws. You might also throw in aircraft, most are designed to fly upside down at least for a short period of time.

So if you've opened your Mikuni already, you have probably noticed there's no fuel float inside. The fuel chamber has very small volume and the fuel inlet control needle (metering needle) is operated by a rubber diaphragm instead of a float.

Here's the trick with this type of carburetor, no fuel can enter the fuel chamber unless engine vacuum is present to pull the rubber metering diaphragm into the metering lever, which lifts the metering needle off it's seat. Once this occurs, fuel can enter the chamber, fill the chamber and equalize the pressure inside the chamber with the atmospheric pressure outside the chamber via the rubber metering diaphragm. For example, if the vent hole in the diaphragm cover plate is covered over using a piece of tape, this carburetor won't work.

Anyway, once pressure on both sides of the diaphragm is equalized, the fuel inlet needle (metering needle) returns to it's seat and restricts inlet fuel from entering the fuel chamber.

So notice the little spring that presses the metering arm against the metering needle, this tiny spring holds the needle in against the seat to restrict fuel entry but if fuel inlet pressure is great enough the force of the spring will be overcome. So happens, we measure this pressure as a reference and it's a pretty good approximation for setting a consistent metering diaphragm vacuum response curve to carefully balance the pressure internally and externally of the fuel chamber and control fuel delivery based on the vacuum from the engine. If the metering arm is bent, mangled or the spring is distorted or incorrect strength, pop-off pressure won't be at specification.

So your pop is the amount of pressure on the fuel supply that forces the metering needle to pop off it's seat and allow fuel to enter the chamber. If it's not at least close to spec within some reasonable range, your carburetor won't fuel correctly, might not even fuel at all or overfuel and fuel-flood the crankcase.

I'm not sure, you didn't say but hopefully you have downloaded the Mikuni Super BN carburetor owners manual. Keep in mind it's an English translation and is a little rough around the edges and is written such that it applies to more than just a few applications where this type of carburetor was used. Thus it's not literal to your application, consider it more of a quickie guide to help explain how the carburetor works.

All specific detail of the calibration values were provided by Seadoo, since Seadoo engineers tuned the Mikuni to operate on their specific application. The office flower who translated the Japanese document probably never even met any Seadoo engineers (my guess) nor did she have a chance to date any.

Further, even the presence of the factory flame arrestor air box is important to the calibration, if you were to remove and leave off the airbox, you would discover the carburetor is calibrated too lean. The reason is, the flame arrestor air box has an effect on the amount of vacuum applied to the carburetor venturis and and thusly, metering diaphragm.

Enough for now, I've gotta go. Any questions just ask.

 

[soccerdad]

Yep. :agree: What he said... Awesome post sportster :cheers:

 

[Mickirig1]

You should have some kind of industrial part supply stores around Austin. They should have a fairly cheap 0 - 50 pound gauge. Buy some 1/4 in rubber tubing, a T and the metal part with shrader valve in it from a tire fill spout. Tire shop might just give you an old one.Hook up a bike tire air pump to metal end other end goes on pulse line with gauge connected to middle of tubing with the T piece. Or just order the $25 from OSD.com

 

[Sportster-2001-951C-Stock]

The OSD pump seems very reasonable considering the running around hassle if you don't already have the box of fittings needed to fit about anything.

There's a Grainger store in South Austin, east side.

I guess NAPA should have all the fittings, or they used to but Austin North NAPA pretty much sucks. Make up a jet pump pressure tester then add an extra fitting for testing carburetors. Foot bike pump from Academy or Wal-Mart has a gauge on it already but sometimes they don't hold pressure like you need for a jet pump but you might get by.

 

[Mickirig1]

Grainger has about everything for anything! You can get fuel line and stainless clamps from them. Before carb fuel filters. Any tool you can think of.

 

[Sportster-2001-951C-Stock]

Grainger has about everything for anything! You can get fuel line and stainless clamps from them. Before carb fuel filters. Any tool you can think of.

True that, they do. I still like the OSD idea, I bet OSD's pump would work great for 10psi pressure testing the jet pump too.

 

[Mickirig1]

We have some much available around here. It's an industrial area for along time. Give me a few days I can find you anything!

 

[mikidymac]

Harbor Freight is also good for cheap pressure gauges.

 

[Frank_Hassa]

Here's a pic of the pop-off tester from OSDParts.com. Their ordering/checkout page doesn't allow you to see the shipping charges until after you enter your payment information.

Here's a 0- 30 psi gauge I found on eBay for $6.00, w/ free shipping.

http://www.ebay.com/itm/1-8-NPT-Air...auge-0-30-PSI-Side-Mount-2-Face-/252169936247

Today I read the SBN manual that describes the operation of the Mikuni carbs, so things are starting to "gel".

Questions:

Do I have a primer port to worry about. It says to plug or cap if it's not being used.

How so I know which of the 3 Mikuni carbs my 1997 Seadoo GSX has (38, 42, 44...)?

Where do I find the spec for pop-off pressure?

 

[mikidymac]

The Seadoo use a taper bore not a true 42 or 44 like the standard SBN. I think yours are a 40-42 but not sure the classify it as a 40i. Pop off range is 23-43.

 

[Sportster-2001-951C-Stock]

1997 GSX - Please measure the pop pressure before you disassemble the carburetors, hopefully they both measure very close to the same result. Report back, so I can update/correct my notes.

Then based on the mikuni chart pop would occur at 38psi for the 80 gram spring in a 1.5 orifice seat.

I'll be interested to learn what you discover!

 

[Mickirig1]

You can find the repair manual for your ski on the Internet for free. Most carb posting's have charts with pop off pressures listed for your carb, ski and year. We are here to help. Don't get frustrated. It was all new to us at some point.

 

[garrettb]

I thought about building a pop off tester for my ski, but all said and done, it was easier to buy a Mityvac. It only cost me $55, and I can pull vacuum or pressurize. Maybe I'll use it to bleed brakes one of these times.

http://www.tooltopia.com/mityvac-mv8500.aspx

 

[soccerdad]

I thought about building a pop off tester for my ski, but all said and done, it was easier to buy a Mityvac. It only cost me $55, and I can pull vacuum or pressurize. Maybe I'll use it to bleed brakes one of these times.

http://www.tooltopia.com/mityvac-mv8500.aspx

Yep those are great tools. And work REALLY well to pull fuel in from the tank to the carbs and to check leaks in the fuel supply system.

 

[wfo speedracer]

I have owned Seadoo popoff testers, Mikuni popoff tester and several homemade ones, the best one I have run across is one a friend gave me, it is used in the prosthetic industry, I am not sure what for , it looks similar the one in the pic below but has a release valve you press to bleed off the pressure. The one in the pic below is used in the model aviation to check retract landing gear and goes to 100 psi, it runs about $35.00 and is probably the last one you would ever need to buy.


http://www.ebay.com/itm/Robart-Manu...4G-/172252537926?_trksid=p2141725.m3641.l6368

Tower hobbies also carries this as well as several other hobby shops.

http://www3.towerhobbies.com/cgi-bin/WTI0001P?I=LXET26&P=8